A frequency modulated signal demodulator (referred to as FM demodulator hereafter) is used to recover data transmitted by frequency modulating a carrier. Many types of FM demodulators have been developed heretofore. In those FM demodulators, a so-called pulse count type FM demodulator or an FM demodulator employing a delay circuit has merit in that it can be constituted of digital devices.
The FM demodulator employing delay circuits has a first gate circuit having an "exclusive-OR" function and receiving at its two inputs pulse signals having the frequency of the frequency modulated signal (referred to as FM signal hereafter), delay means for delaying one of the pulses relative to the other, and a low-pass filter (referred to as LPF hereafter) connected to the output of the first gate circuit.
A pulse count type FM demodulator of the above-described type is disclosed in, for example, U.S. Pat. No. 3,778,727. In prior art pulse count type FM demodulators of this type, a shift register or a delay circuit is employed to produce a fixed delay of one of the input pulses applied to the "exclusive-OR" gate (referred to as EX-OR gate hereafter). This fixed delay is equivalent to a phase shift of the delayed pulse proportional to the frequency. An output pulse formed by the input pulses appearing at the transition instants of the non-delayed input pulse and having a fixed duration is obtained at the output of the EX-OR gate. The LPF connected to the output of the EX-OR gate supplies the average value of the output pulse, which average value is proportional to the phase shift between the delayed and the non-delayed input pulses and, consequently, is proportional to the frequency of the modulated signal. The fixed delay can be realized by means of the delay circuit.
Referring now to FIGS. 1, 2 and 3, a typical example of the conventional pulse count type FM demodulator will be briefly described. As shown in FIG. 1, the example of the conventional pulse count type FM demodulator comprises an input terminal 11, a first wave-shaping circuit 12, an EX-OR gate 13, a delay circuit 14 and an LPF 15. The first wave-shaping circuit 12 is coupled to the input terminal 11. The EX-OR gate 13 is coupled at its first input 13a to the output of the first wave-shaping circuit 12. The delay circuit 14 is coupled between the output of the first wave-shaping circuit 12 and a second input 13b of the EX-OR gate 13. The LPF 15 is coupled to the output of the EX-OR gate 13.
An input FM signal Sfm is supplied to the first wave-shaping circuit 12 from the input terminal 11. The first wave-shaping circuit 12 shapes the input FM signal Sfm into a first pulse signal P1 with a correct rectangular waveform signal, as shown in FIG. 2. FIG. 2 shows signals in the demodulator of FIG. 1 and time relations thereof. In FIG. 2, the left portion shows the signals with a relatively low frequency. The right portion of FIG. 2 shows the signals with a relatively high frequency. The first pulse signal P1 output from the first wave-shaping circuit 12 is supplied to the first input 13a of the EX-OR gate 13.
Further, the first pulse signal P1 is supplied to the delay circuit 14. The delay circuit 14 comprises an integration circuit 16 and a second wave-shaping circuit 17 coupled in series with each other. The second wave-shaping circuit 17 as well as the first wave-shaping circuit 12 are coupled to a power supply source 20 for receiving a prescribed power supply voltage Vcc. The first and second wave-shaping circuits 12 and 17 are further coupled to a ground potential source 21 having a ground potential Vg.
The integration circuit 16 comprises a resistor 18 and a capacitor 19. The resistor 18 is coupled between the output of the first wave-shaping circuit 12 and the input of the second wave-shaping circuit 17. The capacitor 19 is coupled between the right end of the resistor 18, in the drawing, and the ground potential source 21.
The first pulse signal P1 is integrated by the integration circuit 16 so that an integration signal Si, as shown in FIG. 2, is obtained at the right end of the resistor 18. The integration signal Si starts to increase gradually with a predetermined integration curve at the leading edge of the first pulse signal P1. The integration signal Si starts to decrease gradually with the predetermined integration curve at the trailing edge of the first pulse signal P1.
The integration signal Si is supplied to the second wave-shaping circuit 17. Now it is assumed that the first and second wave-shaping circuits 12 and 17 are constituted by using C-MOS devices (Complementary Metal Oxide Semiconductor devices), and the power supply voltage Vcc of 5.0 volts is supplied to the FM demodulator. Then the wave-shaping circuits 12 and 17 have a threshold level Vth at an intermediate level of the power supply voltage Vcc, i.e., 2.5 volts.
The output level of the second wave-shaping circuit 17 changes to the power supply voltage Vcc of 5.0 volts when the integration signal Si exceeds the threshold level Vth, i.e., 2.5 volts. The output level of the second wave-shaping circuit 17 changes to the ground potential of the ground potential source 21, i.e., 0 volts, when the integration signal Si drops below the threshold level Vth, i.e., 2.5 volts. Thus, a second pulse signal P2, as shown in FIG. 2, is obtained from the second wave-shaping circuit 17.
The second pulse signal P2 has a pulse shape equivalent to the first pulse signal P1 output from the first wave-shaping circuit 12, but delayed for a period Td, as described later. The second pulse signal P2 thus delayed relative to the first pulse signal P1 is supplied to the second input 13b of the EX-OR gate 13. The EX-OR gate 13 carries out the "exclusive-OR" operation for the first and second pulse signals P1 and P2 supplied to the first and second inputs 13a and 13b thereof. As a result of the "exclusive-OR" operation, a third pulse signal P3, as shown in FIG. 2, is obtained from the EX-OR gate 13.
The delay time Td of the second pulse signal P2 from the first pulse signal P1 is defined by a period T1 from the instant ta or tb corresponding to the leading or trailing edge of the first pulse signal P1, to the instant tc or td at which the integration signal Si crosses the threshold level Vth in the course of increasing or decreasing. The above period T1 is kept uniform, if the minimum and maximum levels Vmin and Vmax of the integration signal Si are fixed. Another period T2 from the instant tc or td to the instant tb or ta varies in accordance with the frequency of the input FM signal Sfm. When the frequency of the input FM signal Sfm is relatively low, as shown by the left portion in FIG. 2, the third pulse P3 output from the EX-OR gate 13 has a wide OFF pulse period. When the frequency of the input FM signal Sfm is relatively high, as shown by the right portion in FIG. 2, the third pulse P3 output from the EX-OR gate 13 has a narrow OFF pulse period.
The third pulse P3 is supplied to the LPF 15. The LPF 15 smoothes signals input thereto so that a signal Sa having an average level of the third pulse P3 is obtained from the LPF 15. An amplitude voltage Va of the output signal Sa equivalent to the average level of the third pulse P3 varies dependent upon the duration of the OFF pulse period of the third pulse P3. The signal Sa output from the LPF 15 is obtained as an FM demodulation signal from an output terminal 22 of the FM demodulator, which is coupled to the LPF 15.
Thus, the amplitude Va of the FM demodulation signal Sa is relatively small, as shown by the left portion in FIG. 2, when the frequency of the input FM signal Sfm is relatively low. The amplitude Va of the FM demodulation signal Sa is relatively large, as shown by the right portion in FIG. 2, when the frequency of the input FM signal Sfm is relatively high.
The conventional FM demodulator of FIG. 1, however, has a drawback in which the FM demodulation signal Sa obtained thereby has an insufficient response characteristic to the frequency of the input FM signal Sfm. This is because the delay time Td of the second pulse signal P2 from the first pulse signal P1 varies in accordance with the frequency of the input FM signal Sfm, in spite of the fact that the delay time Td must be kept uniform.
As shown in FIG. 2, the amplitude of the integration signal Si becomes large when the frequency of the input FM signal Sfm is low. Thus, the minimum and maximum levels Vmin and Vmax of the integration signal Si go away from the threshold level Vth. On the other hand, the amplitude of the integration signal Si becomes small when the frequency of the input FM signal is high. Thus, the minimum and maximum levels Vmin and Vmax of the integration signal Si approach the threshold level Vth.
The minimum and maximum levels Vmin and Vmax define the initial values at the start of the integrating operations. The minimum and maximum levels Vmin and Vmax of the integration signal Si distant from the threshold level Vth make the delay time Td longer, as shown by the left portion in FIG. 2. On the other hand, the minimum and maximum levels Vmin and Vmax of the integration signal Si close to the threshold level Vth make the delay time Td shorter, as shown by the right portion in FIG. 2.
Thus, the conventional FM demodulator has a response characteristic, as shown in FIG. 3. As shown in FIG. 3, the demodulation output voltage Va of the FM demodulation signal Sa does not respond linearly to the frequency of the input FM signal Sfm.
In order to avoid the drawback, another prior art FM demodulator, for example, U.S. Pat. No. 4,435,682 has been proposed. FIG. 3 of the patent discloses an FM demodulator employing delay circuits. In the FM demodulator of the patent, a differentiation waveform signal is obtained by an exclusive-0R operation for signals supplied from both an input circuit and an output circuit. Thus, the construction of the FM demodulator of the patent is very complicated. Further, the FM demodulator of the patent is difficult for maintaining timings of signals in stable relations.